Pest consumption and niche separation of common immigrant and agrobiont spider species in semi-desert wheat fields

Natural enemy populations are important in suppressing outbreaks of pest populations. However, disturbances in agricultural fields due to crop managment do not allow maintenance of sustainable predator populations. Therefore, alternative habitats that provide reproduction sites, shelter and alternative prey, are important for natural enemies that immigrate into the agricultural fields during the crop season. In the desert agroecosystem of Israel there are large ecological differences between natural habitats and crop fields. As a result, the high-productivity agricultural fields attract predators from the surrounding natural desert environment during the crop season. The two most common web building spiders present in wheat fields during the cropping season are an agrobiont species, Alioranus pastoralis (Linyphiidae), and an immigrant species, Enoplognatha sp. (Theridiidae). These species share the same niche, and may be competing for the same resources. We found that the agrobiont species has a prey preference for Collembola, an abundant non-pest prey, , while the immigrant Enoplognatha consumed aphids (pest species). These results clearly demonstrate the potential role of immigrant spiders in biological control of pest species in winter wheat fields in Israel. However, why do immigrant species consume more of the pest than do the agrobionts? Two alternative hypotheses are interspecific competition and specific prey preferences. These questions were investigated using DNA based gut-content analysis and monitoring the spider and pest densities and web locations throughout the crop season.